This project seeks to delineate the role of an understudied component of ECE contexts- the role of peers. Based on theoretical frameworks arguing for the central role of peers in affecting both children and adults, this study will specify and test a conceptual model delineated specific paths through which peer effects operate. Specifically, it is hypothesized that peer skills and behaviors within a classroom will predict shifts in individual child skills through the preschool year, and will also predict shifts in teacher instructional practices, which in turn are hypothesized to predict individual child functioning in kindergarten. As the first study to assess the potential for peers to affect both individual children and teachers, this research has the potential to provide essential insights into classroom composition effects, which in turn will inform ECE policies regarding targeted versus universal programs and efforts to improve ECE quality and children's development. This project will employ a broad array of dissemination and collaboration strategies in order to increase connections and collaborations in the ECE field, and to assure a bidirectional transfer of knowledge and ideas between policy, practice, and research. This project seeks to expand knowledge of peer effects and address issues of central concern to early education decision makers.